PageMethods failed on IIS 7.5 of Win 2008 R2 box

I have the following ASP.NET 4.0 code and it is running well on IIS 6.0 of the Win 2003 box. But When I pushed it to the Win 2008 R2 box(IIS 7.5 classic mode pool), it failed to onAddrLoadError. I am using Web Farms and maybe it is the web farms which caused this problem. I am also using SSL on the load balancer.

<asp:ScriptManager开发者_StackOverflow ID="ScriptManager1" EnablePageMethods="true"    runat="server"></asp:ScriptManager>

function AddEdit_onclick(id) 
{
    PageMethods.GetProduct(id, onAddrLoadSuccess, onAddrLoadError);
}

Any ideas?

I just installed the .NET 4.0 hotfix for win 2008 and it works.